Applicant's commonly assigned U.S. Pat. No. 4,778,631 discloses a method and apparatus for achieving continuous foaming of a variety of polymeric materials having a wide range of viscosities, including viscosities extending from about 50,000 up to above 1,000,000 centipoise. The disclosure of that patent is hereby expressly incorporated by reference in its entirety.
According to the teachings of this patent, force feeding of a gas and a high viscosity polymeric material into and through a low energy input mixer, with a low pressure drop across the mixer, avoids premature foaming or an unacceptable temperature rise of the high viscosity material. This mixing operation produces a pressurized solution of high viscosity polymeric material with a substantially uniform dispersion of gas bubbles therein.
Throughout the specification and claims of this application, the term "solution" is used to describe the liquid polymer containing a dissolved gas supplied under high pressure to a dispensing device, which creates a foamed polymeric structure when dispensed at atmospheric pressure. The term "solution" as used in the specification and the claims of this application is intended to define and encompass the broader generic definition of solution which is a homogenous mixture of a gas and a molten or liquid polymer, whether or not the gas molecules are in fact dissolved or dispersed among the polymer molecules.
After mixing, the foamable solution is then transferred under pressure to a dispensing device such as a valved nozzle, from which device the solution is dispensed through an outlet to atmospheric pressure. Upon emerging from the outlet of the dispenser, the gas evolves from the solution in the form of small bubbles which enlarge to cause the polymeric material to expand volumetrically. The resultant product in an uncompressed state becomes a homogeneous foam having gas pores or cells, which may be of various forms including both open and closed cells, and which are substantially evenly distributed throughout the polymer. As the polymer material cools or cures, a permanent homogeneous foam is created.
While a foamed high viscosity polymer may be suitable for use in a variety of applications, for instance as an adhesive, or a sealant or a coating, these foamed polymers are particularly suitable for use as a gasketing material. This is due to the fact that a gasket made from a foamed high viscosity polymer may be conveniently formed-in-place by extrusion, at a relatively low cost when compared to other methods of placement and securing gaskets, such as, for instance, adhesive mounting of a pre-formed or molded gasket. Additionally, the resulting formed-in-place gasket has a number of desirable physical characteristics related to the high viscosity of the polymer, such as increased durability, increased resiliency and improved memory, or recovery from compression.
Unfortunately, while formed-in-place gaskets offer a number of advantages over pre-formed, adhesively secured or mounted gaskets, their usefulness is limited by the maximum aspect ratio that can be achieved with an extruded bead of the material. The term "bead" in the context of this application refers to a continuous line of material extruded through a nozzle. Aspect ratio represents the ratio of the height of the bead to its width. Many gasket applications require bead dimensions with aspect ratios that are simply not obtainable with conventional extruding or dispensing methods or devices.
One of the reasons that higher aspect ratios are thought to be unobtainable by extrusion relates to the problem of slumping, or the tendency of an extruded bead to spread out or slump under its own weight before the bead sets-up or solidifies. The greater the amount of material extruded in an effort to achieve a desired height, the greater the weight of material that must be supported, and slumping occurs. Eventually, a point is reached where the amount of material that must be extruded in order to achieve a desired dimension becomes so great that the cost of material necessary to produce a formed-in-place gasket is higher than the cost of producing the gasket in another manner.
It is therefore an object of this invention to provide a formed-in-place gasket with improved bead characteristics, particularly an increased aspect ratio.
It is another object of the invention to provide a cost effective, formed-in-place gasket suitable for applications requiring relatively high aspect ratios.
It is still another object of this invention to provide a method and apparatus capable of extruding a bead of high viscosity polymeric material with an increased aspect ratio, and in a manner which results in a cost savings in material.